Regarding gasoline engines, the harmful components in the exhaust gases have been decreasing securely by means of the strengthening of automotive exhaust-gas regulations and the developments of technologies that can cope with them. On the other hand, regarding diesel engines, the conversion of the exhaust gases is more difficult than in the case of gasoline engines, because of such an usual circumstance that the harmful components have been emitted as PM (i.e., carbon fine particles, sulfur-system fine particles such as sulfates, and high-molecular-weight hydrocarbon particulates (or SOF), and the like).
Hence, mesh-closed type honeycomb bodies (or diesel particulate filters (hereinafter, being referred to as “DPF”)), which are made of ceramic, have been known heretofore. The DPF is one which is completed by closing meshes at the opposite ends of the cellular openings of a ceramic honeycomb structural body alternately in a checkered manner, for instance. It comprises; inlet-side cells that are mesh plugged on the exhaust-gas downstream side; outlet-side cells that neighbor the inlet-side cells and are mesh plugged on the exhaust-gas upstream side, and porous cellular partition walls that demarcate the inlet-side cells and the outlet-side cells; and captures PM by filtering exhaust gases with the pores in the cellular partition walls.
Moreover, as set forth in Japanese Examined Patent Publication (KOKOKU) Gazette No. 7-106,290, a filter catalyst has been developed recently, filter catalyst in which a catalytic layer that is completed by supporting a catalytic metal, such as platinum (Pt), onto alumina, or the like, is formed on the surface of the cellular partition walls of DPF. In accordance with this filer catalyst, since captured PM are oxidized and then burned by means of the catalytic metal's catalytic reactions, it is possible to continuously recover the filter catalyst by letting them burn simultaneously with their capture, or in succession to their capture.
Further, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 9-94,434, there is set forth a filter catalyst in which a catalytic layer is formed not only on the cellular partition walls but also within the pores in the cellular partition walls. By forming a catalytic layer within the pores as well, the contact probability between PM and catalytic metal is enhanced, and thereby it is possible to burn PM, which have been captured within the pores, by means of oxidation. Moreover, in the catalytic layer, NO in exhaust gases is oxidized to generated NO2 with high oxidizing activity, and thereby the oxidation reactions of PM by means of this NO2 are also expected.
And, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 9-125,931, there is set forth a filter catalyst in which a noble metal, and an NOx storage material are contained without ever forming any coating layer. By thus containing a noble metal and an NOx storage material, it is possible to store NOx in lean atmosphere, and it is possible to release stored NOx by turning the lean atmosphere into rich atmosphere intermittently and then convert them by means of reduction. Therefore, it is possible to convert PM and NOx in exhaust gases efficiently.
However, since a reducing agent that is supplied in rich atmosphere is consumed at first on the upstream side of filter catalyst, a concentration distribution arises between the upstream side and the downstream side, or between the inlet-side cells and the outlet-side cells. Therefore, in a filter catalyst on which an NOx storage material is supported uniformly, a distribution arises in the reaction magnitude of NOx with the reducing agent on the NOx storage material, and accordingly no efficient NOx conversion can be done. Specifically, such circumstances occur that: although the NOx-storage-material amount is great, the reducing agent is less; on the contrary, although the NOx-storage-material amount is less, the reducing agent is great.
Hence, in Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-207,836, there is proposed a filter catalyst on which a supporting amount of a catalytic substance is distributed greatly on the inlet side without ever forming any coating layer. In accordance with the filter catalyst set forth in this gazette, it is possible to carry out the conversion of exhaust gas more actively on the inlet side, where the concentrations of harmful substances such as PM are high, by distributing a supporting amount of a noble metal greatly on the inlet side. Moreover, it is possible to carry out the NOx reduction conversion with high NOx reduction conversion efficiency in a well-balanced manner as a whole by distributing a supporting amount of an NOx storage material greatly on the inlet side where the concentration of a reducing agent is high.
In order to improve an NOx conversion ratio by making an NOx storing amount greater, it is required that an NOx storage material be supported greatly. However, when having a filter substrate directly contain an NOx storage material greatly without forming any coating layer, such a case often arises that the reaction between the NOx storage material and the substrate has occurred, and thereby it is not possible to have it contain a required amount of the NOx storage material. Moreover, there is also such a problem that the activity is low, compared with a filter catalyst in which a catalytic layer on which a catalytic component is supported is formed on a coating layer that comprises a porous support such as alumina.    Patent Literature No. 1: Japanese Examined Patent Publication (KOKOKU) Gazette No. 7-106,290;    Patent Literature No. 2: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 9-94,434;    Patent Literature No. 3: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 9-125,931; and    Patent Literature No. 4: Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2001-207,836